Trench Calculator (Excavation and Backfill)
This trench calculator works out how much soil you dig out, how much gravel bedding goes in, and how much backfill comes back, for a pipe, drain or cable trench.
A 50 ft trench 18 in wide and 3 ft deep removes 8.33 cubic yards of soil, which swells to about 10.4 yd³ loose. Subtract 4 inches of gravel bedding (0.93 yd³) and a 4-inch pipe, and you need about 7.2 yd³ of backfill.
Your numbers
Result
How much soil does a trench hold?
A trench is a long, narrow box: its volume is length times width times depth. From there you take away what goes back in as bedding or pipe, and what is left is backfill.
bedding = length × width × bedding depth
backfill = trench volume − bedding − pipe volume
pipe volume = π × (diameter ÷ 2)² × length
Example: 50 ft × 1.5 ft × 3 ft = 225 cu ft = 8.33 yd³. Dug soil swells about 25%, so haul 10.42 yd³. Bedding of 4 in is 25 cu ft (0.93 yd³, about 1.3 tons of gravel). A 4 in pipe takes 4.4 cu ft. Backfill is 225 − 25 − 4.4 = 195.6 cu ft, or 7.25 yd³.
Trench size to soil removed (50 ft long)
| Width × depth | In-place yd³ | Loose yd³ (25% swell) |
|---|---|---|
| 12 in × 2 ft | 3.70 | 4.63 |
| 18 in × 3 ft | 8.33 | 10.42 |
| 24 in × 4 ft | 14.81 | 18.52 |
| 36 in × 5 ft | 27.78 | 34.72 |
Before you dig
- Call 811 to have buried utilities marked.
- Trenches 5 ft deep or more generally need protective systems under OSHA rules; follow them and local code, and never work in an unprotected deep trench.
- Backfill in layers and compact; the fill dirt calculator gives you quantities if you need to import soil.
- For larger digs use the excavation calculator; for bedding stone see the gravel calculator.
Frequently asked questions
How do I calculate the volume of a trench?
Multiply length by width by depth in feet to get cubic feet, then divide by 27 for cubic yards.
How much does soil swell when dug?
Loose soil takes more space than in the ground, often 20 to 30% more depending on the soil. The default here is 25%.
How much gravel do I need for pipe bedding?
Bedding volume is length × trench width × bedding depth, then convert to yards or tons. This calculator includes the tons at 1.4 per yd³.
Do I need extra backfill for compaction?
Often a little, because compacted soil takes less room. Add a margin or measure after compacting a first layer.
Are deep trenches regulated?
Yes. OSHA has trench safety requirements, and local rules may add more. Treat deep trenches as a job for trained crews.
Sources and further reading
- Trenching and excavation safety — Occupational Safety and Health Administration
- NIST Special Publication 811: conversion factors — National Institute of Standards and Technology
These references support the general guidance on this page. The calculator applies the standard formulas shown above to the numbers you enter.
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Related calculators
. Estimates only. Always confirm quantities with your supplier or a qualified contractor before ordering, and add extra for waste and site conditions.